1. Field of the Invention
The present invention relates to a liquid crystal display element of dot matrix system, and more particularly, to a liquid crystal display element that is superior in flexibility.
2. Description of the Related Art
In the future, electronic paper that can hold a display without a power supply and can electrically rewrite contents of the display is expected to become rapidly widespread. Electronic paper implements a book, a magazine, newspaper, etc., which are conventionally paper printed matters, by using an apparatus that can electrically rewrite a display, and has superior characteristics of paper printed matters, such as thinness, lightweight, and easiness to look. Electronic paper is superior to paper printed matters in a point that contents of a display can be rewritten. Accordingly, electronic paper is not discarded like paper printed matters. Therefore, electronic paper greatly contributes to reductions in the consumption of paper resources if it becomes widespread as an alternative to paper printed matters. This is considered to be very useful also from the viewpoint of environmental protection.
As applications of electronic paper, an electronic book, an electronic newspaper, an electronic poster, an electronic dictionary, etc. are considered. As characteristics demanded for electronic paper, the following (1) to (5) and the like exist.    (1) electrical rewritability of display data    (2) ultra-low consumption power    (3) easy on the eyes, and difficult to be tired (very easy to look)    (4) easy to carry (lightweight and easy to tote)    (5) as thin as paper and bendable (lightweight and flexible)
Electronic paper has been studied and developed by using an electrophoresis system, a twist ball system, a liquid crystal display, an organic EL (electro-luminescence) display, etc.
The electrophoresis system is a system for moving charged particles in the air or a liquid. The twist ball system is a system for rotating two-color-coded charged particles. The organic EL display (organic electro-luminescence display) is a self-luminous display unit having a structure where a plurality of thin films made of an organic material are sandwiched by negative and positive electrodes. The liquid crystal display is a non-self-luminous display unit having a structure where a liquid crystal layer is sandwiched by a pixel electrode and an opposed electrode.
Electronic paper implemented with the liquid crystal display has been studied and developed by using cholesteric liquid crystal of selective reflection type having bistability. Here, the bistability is a nature that a liquid crystal exhibits stability in two different alignment states, and the cholesteric liquid crystal has a nature that two stable states such as planar and focal conic states are maintained for a long time even after an electric field is removed. For the cholesteric liquid crystal, incident light is interfered and reflected in the planar state, whereas incident light is transmitted in the focal conic state. Accordingly, a liquid crystal panel using the cholesteric liquid crystal as a liquid crystal layer, the lightness/darkness of light can be displayed with the selective reflection of incident light in the liquid crystal layer. This eliminates the need for a polarization plate. The cholesteric liquid crystal is also called chiral nematic liquid crystal.
Since the cholesteric liquid crystal reflects a color with the interference of liquid crystal, a color display can be made only by stacking the liquid crystals. Therefore, a liquid crystal display system using the cholesteric liquid crystal (here, referred to as a cholesteric liquid crystal system for the sake of convenience) is overwhelmingly superior in a color display to other systems such as the above described electrophoresis system, etc. With the other systems, color filters tinted in three colors must be arranged for each pixel. Therefore, its brightness is one-third of the cholesteric liquid crystal system. Accordingly, for the other systems, improving the brightness is a significant challenge to implement electronic paper.
As described above, the cholesteric liquid crystal has an advantage that a color display is easy, but its most significant challenge is to impart flexibility, which is the characteristic of electronic paper.
For a liquid crystal display element, a uniform cell of a gap of several pm is required. Normally, a cell is formed by sandwiching a liquid crystal layer (several μm) between upper and lower glass substrates. For a normal liquid crystal panel of TN (Twisted Nematic) type or STN (Super Twisted Nematic) type, also some liquid crystal display element (plastic liquid crystal) using a film substrate made of transparent special resin is implemented. The plastic liquid crystal element can be reduced in thickness and weight compared with the liquid crystal of a glass substrate, and also has high endurance and a high strength against bending. Accordingly, the plastic liquid crystal is freely bendable like paper, and suitable for electronic paper.
Here, a conventional configuration for implementing a uniform cell gap of a liquid crystal panel is described.
FIG. 1 is an exploded view of the cell structure of a liquid crystal display element of a dot matrix structure, which implements a uniform cell gap by using a support spacer.
The liquid crystal display element shown in this figure has a structure where a liquid crystal layer is sandwiched between a first substrate (lower substrate) 1 and a second substrate (upper substrate) 2. The liquid crystal layer is composed of a seal member 3, adhesive supports 5, etc. On the surface of the first substrate 1, a plurality of transparent column electrodes (not shown) are formed. Additionally, on the back of the second substrate 2, a plurality of transparent row electrodes (not shown), which vertically intersect the column electrodes, are formed. On the side of the liquid crystal layer on the first substrate 1 on which the column electrodes are formed, the seal member 3 is formed.
The seal member 3 is an adhesive of thermal hardening type or UV (ultraviolet) hardening type manufactured in a printing process, and configures the periphery of the liquid crystal layer. In the center of one side 3a of the seal member 3, an opening is provided, and both of its ends extend to form an inlet 4 of liquid crystal. Namely, a portion of the seal member 3 is the inlet 4 of the liquid crystal, via which the liquid crystal is poured into a region enclosed by the seal member 3.
Within the region enclosed by the seal member 3, a plurality of adhesive supports 5, which serve as spacers of the liquid crystal layer, are formed. These adhesive supports 5 are formed at the four corners of each pixel of the liquid crystal layer.
The adhesive supports 5 are members that take the shape of a cylinder, and can be bonded to the second substrate 2. Therefore, if the first substrate 1, on which the seal member 3 and the adhesive supports 5 are formed, and the second substrate 2 are stacked, the first substrate 1 and the second substrate 2 are adhered with the seal member 3 and the adhesive supports 4. The seal member 3 is, for example, a member that is hardened by being heated.
With the liquid crystal display element having the above described configuration using the cholestric liquid crystal of selective reflection type for the liquid crystal layer, a portion between pixels, in which an electrode is not provided in an upper or lower opposed position, stays lit up. Therefore, to improve the contrast of a pixel by preventing the portion from staying lit up, a black matrix 6 is formed on the second substrate 2. This black matrix 6 is a grid-shaped pattern corresponding to portions (four sides of a pixel) of a liquid crystal layer, in which an electrode (row or column electrode) is not arranged downward or upward.
In the liquid crystal display element having the above described configuration, the adhesive supports 5 serve as spacers, whereby a width (cell gap) between the first substrate 1 and the second substrate 2 is kept uniform.
A support like the adhesive support 5 can be formed, for example, with patterning using a photolithography disclosed by Japanese Unexamined Utility Model Application Publication No. 58-13515, or Japanese Unexamined Patent Application Publication No. H8-76131.
In the liquid crystal display element having the above described configuration, cholesteric liquid crystal is poured from the inlet 4, whereby a cholesteric liquid crystal display element of selective reflection type can be implemented. However, to the cholesteric liquid crystal display element of selective reflection type, flexibility cannot be imparted only by implementing a uniform cell gap.
Because liquid crystal is a liquid, it flows by force applied by operations such as bending of a liquid crystal panel, or pressing of its display surface, and a display state varies. Since the display of the liquid crystal panel of TN type or STN type continues to be electrically driven, its display state can immediately revert to the original state even if it varies. However, for the cholesteric liquid crystal having the memory property of a display, its display does not revert to the original state until it is again driven.
A method for forming a support like the adhesive support 5 shown in FIG. 5 in the cholesteric liquid crystal display element is disclosed, for example, by Japanese Published Unexamined Patent Application Publication No. 2000-146527. The invention disclosed by this publication mainly aims at ensuring the uniformity of cell gaps, and does not aim at holding the memory property of a cholesteric liquid crystal display element when a liquid crystal panel is bent or its display surface is pressed.
To apply the cholesteric liquid crystal of selective reflection type to electronic paper, implementing a structure with which a display does not vary even if electronic paper is pressed or bent was the greatest challenge.
As a result of creating a liquid crystal cell having the support structure shown in FIG. 1 with a film substrate of 0.125 μm, its display varied only by being held by hand. To prevent the display from varying, a robust housing was required for the support structure of the liquid crystal cell, and it was impossible to apply this liquid crystal cell to electronic paper having flexibility.
The present inventor ascertained the mechanism, with which the display of a liquid crystal cell using cholesteric liquid crystal having a conventional support structure varies with the pressing force of its display surface, by experiment. This mechanism is disclosed by PCT Application No. PCT/JP2004/013380 previously filed by the present inventor.
The cause of the above described display variance is the fluidity of liquid crystal (cholesteric liquid crystal), which is resultant from pressing force applied to a display surface, or the bending of a liquid crystal cell, and the problem of the display variance can be solved by suppressing this fluidity. A spacer structure using a cylinder or a square support cannot suppress the fluidity of liquid crystal. Also a support having a stripe structure for the uniformity of cell gaps is proposed. With this structure, however, liquid crystal easily flows.